This invention relates to multiple beam antennas such as those used in satellite communication systems for illuminating a substantial portion of the earth's surface and, more particularly, to a wide-area coverage by spaced-apart high-gain beams including a sequential offsetting of a set of plural high-gain beams along a set of paths which carries each beam around a central location thereof, so as to illuminate areas between locations of high beam intensity for improved uniformity of illumination.
A multiple beam antenna array is employed to provide high gain coverage of a wide area; the beams are spaced apart so as to increase the areas of illumination attainable with the limited number of beams. However, the resulting illumination of a reception region, such as a region of the earth having receivers for receiving satellite communications, varies in intensity of received electromagnetic power. Maximum intensity of illumination is received at areas located on center lines of the beams. Generally, lower gain areas occur at a crossover region between beams. The foregoing example of a communication system has been given in terms of a transmission of electromagnetic power from the antenna, with the illumination intensity varying as a function of the antenna radiation pattern. However, its is to be understood that the operation of a multiple-beam antenna is reciprocal so that the foregoing variation in illumination applies also to reception, at a satellite, of transmissions from ground stations wherein the strength of a signal received at the satellite is dependent upon the location of the transmitting site within the antenna radiation pattern.
The requirements of a point to multi-point communications link may require that maximum antenna gain be provided simultaneously in numerous selected directions. This is particularly true in a situation wherein each of the multi-point communications terminals have minimal rf (radio frequency) link capabilities. An example of this situation is a satellite communication system where many small terminals are dispersed across a global coverage area. A single satellite antenna beam of sufficient gain to communicate over the link covers only a small portion of the necessary coverage area. In this situation, multiple beams can be implemented to cover the required coverage area with a matrix of high gain spot beams.
By way of example, each of the constituent beams may be circular with a peak gain at boresight, and with the gain decreasing with angle off boresight. For continuous coverage of a given region, the beams will be located usually so that the coverage pattern of each beam intersects the coverage pattern of its adjacent neighboring beams at a reduced gain, crossover contour, typically several decibels (dB) below beam peak. As an example, a geosynchronous satellite system can provide full hemispherical coverage of the globe with 109 spot beams, each having a 1.9 degree beam diameter as defined by the -4 dB (relative to peak power) gain contour. For this case, any point in the coverage area would be serviced by a beam at a gain level no lower than 4 dB relative to the peak power of a beam. If the locations of the multi-point terminals are distributed across the coverage region, then some receiving stations will benefit from the relatively high illumination intensity of single point antenna gains approaching peak beam gain while other receiving stations will be serviced by the foregoing minimum crossover gain.
A problem arises in that there are situations wherein these differences in gain, or intensity of illumination, cannot be adequately compensated by receiver circuit construction, and system performance will be degraded for receivers located in the minimum antenna gain regions.